Nonwoven fibrous sheet structures

ABSTRACT

This invention relates to a new nonwoven material which has very high Frazier permeability while having substantial hydrostatic head liquid barrier properties. The material is comprised of fibers which are approximately one denier and finer fibers which have sufficient strength properties so as not to need a support scrim. The fabric is quite comfortable because of its breathability, quite soft because of its construction, and protective from liquids from rain to hazardous chemicals.

FIELD OF THE INVENTION

[0001] This invention relates to nonwoven fibrous structures and moreparticularly to breathable fabrics and sheet structures formed by fiberswhich are held together without weaving or knitting.

BACKGROUND OF THE INVENTION

[0002] Nonwoven fibrous structures have been around for many years andtoday there are a number of different nonwoven technologies incommercial use. To illustrate the breadth of nonwoven technologies,paper is probably one of the earliest developed nonwoven fibrousstructures. Nonwoven technologies continue to be developed by thoseseeking new applications and competitive advantages. One broad marketarea that has proven to be highly desirable because of its large volumeand economics is the protective apparel market. This market comprisesprotection from hazardous chemicals such as in chemical spill clean up,from liquids such as blood in the medical field and from dryparticulates or other hazards such as painting or asbestos removal. Thismarket is served by a number of competing technologies.

[0003] Focusing simply on the medical protective apparel market, E. I.du Pont de Nemours and Company (DuPont) makes Sontara® spunlaced fabricswhich are used extensively for medical gowns and drapes and, for certainapplications within the medical field, Tyvek® spunbonded olefin.

[0004] Sontara® spunlaced fabrics have long been used in the medicalfield because of their exceptional performance and comfort. Sontara®spunlaced fabrics for medical protective apparel uses are typicallycomprised staple length polyester fiber hydroentangled with woodpulp.The fabric is finished with a moisture repellent coating to render itstrike through moisture resistant.

[0005] Tyvek® spunbonded olefin is particularly useful in medicalpackaging where it provides valuable advantages such as permittingsterilization in the package. It also is extremely low linting therebyminimizing contamination in the operating room.

[0006] Other technologies that compete in the medical field includecomposite or laminated products. The composite provides a balance ofproperties suitable for the end use. One competitive technology isgenerally called “SMS” in the industry for Spunbond/Meltblown/Spunbond.The basic SMS nonwoven material is described in U.S. Pat. No. 4,041,203with further improvements described in U.S. Pat. Nos. 4,374,888 and4,041,203. The spunbond outer layers are comprised of spunbond nonwovenwhich provides strength but is not able to attain the barrier propertiesof the meltblown inner layer. The technology for making meltblown fibersis swell suited to making fine low denier fibers which are able to havebarrier and breathability but is not suited to obtaining suitablestrength to withstand use as a garment.

[0007] U.S. Pat. Nos. 4,622,259 and 4,908,163 are directed to animprovement over SMS technology by making the meltblown fibers withimproved tensile properties. By providing better meltblown fibers, onemay avoid applying the scrim reinforcement and obtain a lighter weightfabric.

[0008] It is an object of the present invention to provide a furtherimproved nonwoven structure which has a balance of properties which arebetter suited to barrier end uses.

[0009] It is further object of the present invention to provide anonwoven structure that has more substantial barrier and breathabilityproperties compared to currently known barrier materials.

SUMMARY OF THE INVENTION

[0010] The above and other objects of the invention are achieved by aflexible sheet material having a Frazier permeability of at least about70 m³/min-m² and an unsupported hydrostatic head of at least about 15centimeters.

[0011] The invention further relates to a flexible sheet material havinga Frazier permeability of at least about 28 m³/min-m² and an unsupportedhydrostatic head of at least about 30 centimeters.

[0012] The invention also relates to a flexible sheet material having aFrazier permeability of at least about 15 m³/min-m² and a hydrostatichead of at least about 40 centimeters.

[0013] The invention includes a flexible sheet material having a Frazierpermeability of at least about 1 m³/min-m² and a hydrostatic head of atleast about 80 centimeters.

[0014] In another aspect the invention comprises a flexible sheetmaterial comprised of meltspun nonwoven fibers having an average lengthof at least about 4 cm with a cross section of a substantial majority ofthe fibers is less than 70 μm² and the average fiber strength is atleast 275 N/mm².

[0015] In a still further aspect, the invention comprises a flexiblesheet material formed of nonwoven fibers where in the sheet has a basisweight of at least about 13 g/m² and up to about 75 g/m², and whereinsubstantially all of the fibers are continuous meltspun fibers, asubstantial majority by weight of the fibers have a cross section ofless than about 90 microns, and wherein the sheet material has a Frazierpermeability of at least about 1 m³/min-m² and a hydrostatic head of atleast about 25 centimeters.

[0016] The invention further relates to a radiation sterilization stablesheath-core multi-component fiber suited for making a thermally bondednonwoven fabric wherein the core polymer is polyethylene teraphthalateand the sheath fiber is polypropylene teraphthalate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention will be more easily understood by a detailedexplanation of the invention including drawings. Accordingly, drawingswhich are particularly suited for explaining the invention are attachedherewith; however, it should be understood that such drawings are forexplanation only and are not necessarily to scale. The drawings arebriefly described as follows:

[0018]FIG. 1 is a perspective view of a first preferred embodiment formaking the inventive fabric;

[0019]FIG. 2 is a perspective view of a second preferred embodiment formaking the inventive fabric;

[0020]FIG. 3 is a chart illustrating one of the properties of theinventive fiber of the present invention;

[0021]FIG. 4 is second chart illustrating a second property of theinventive fiber of the present invention;

[0022]FIG. 5 is a third chart illustrating a third property of theinventive fiber of the present invention; and

[0023]FIG. 6 is an enlarged cross sectional view of a sheath-corebi-component fiber.

[0024] Detailed Description of the Preferred Embodiment

[0025] Turning now to the drawings there are a number of alternativetechniques for making the inventive materials. In FIG. 1, there isillustrated a first preferred embodiment of a meltspun low denierspinning system, generally referred to by the number 10 for making acontinuous roll of fabric. The system 10 comprises a continuous belt 15running over a series of rollers. The belt 15 includes a generallyhorizontal run under a series of one or more spinning beams 20. In eachspinning beam 20 is provided molten polymer and a large number of verysmall holes. The polymer exits through the holes forming a single fiberat each hole. The fibers are preferably hard yarn fibers which arestrong and resist shrinkage. Typically, hard yarn fibers are made byquenching and drawing the fibers after they are spun so that the polymerchains are oriented within the fiber. It has been found, as will bedescribed below, that hard yarn fibers may also be made by high speedspinning. Such high speed spinning may be the key to suitable fiberproperties as well as suitable productivity to make the fabric pricecompetitive.

[0026] Once the strong fibers have been formed, the fast moving and veryfine fibers are directed to the moving belt 15. This is no small taskdue to the number of fibers and their reactivity to the turbulent airforces in the vicinity. Suitable guides, preferably including airbaffles, are provided to maintain some control as the fibers arerandomly arranged on the belt 15. One additional alternative forcontrolling the fibers may be to electrostatically charge the fibers andperhaps oppositely charge the belt 15 so that the fibers will be pinnedto the belt once they are laid down. The web of fibers are thereafterbonded together to form the fabric. The bonding may be accomplished byany suitable technique including thermal bonding or adhesive bonding.Hot air bonding and ultrasonic bonding may provide attractivealternatives, but thermal bonding with the illustrated pinch rolls 25and 26 is probably preferred. It is also recognized that the sheetmaterial may be point bonded for many applications to provide a fabriclike hand and feel, although there may be other end uses for which it ispreferred that the sheet be area bonded with a smoother finish. With thepoint bonding finish, the bonding pattern and percentage of the sheetmaterial bonded will be dictated so as to control fiber liberation andpilling as well as other considerations. The fabric is then rolled up ona roll 30 for storage and subsequent finishing as desired.

[0027] A second arrangement for making the inventive material of thepresent invention is shown in FIG. 2. In FIG. 2, there is shown a wetlaynonwoven fabric forming system generally referred to by the number 50.The wet lay system 50 includes a foraminous or screen belt 55 runningover a series of rollers. A trough 60 is arranged over the belt 55 todeposit a slurry of liquid and discontinuous fiber thereon. As theslurry moves along with the belt 55, the liquid passes through theopenings in the belt 55 and into a pan 61 (also called a pit). The fiberis randomly arranged and is bonded together at the pinch rollers 65 and66. It should be recognized that there are a number of techniques forbonding the fibers together including through air bonding, resin bondingas well as other suitable bonding techniques. The nonwoven fabric isthen rolled up on a roll 70 for storage or subsequent finishing.

[0028] The fiber in the inventive fabric is a small denier polymericfiber which forms numerous, but very small pores. Putting small denierfiber in a fabric to obtain high barrier is generally known in the artand is not new. However, it has been found that when hard yarn meltspunmicrofibers are used to create a nonwoven fibrous structure, theresulting fabrics have extraordinarily high Frazier permeability. Thisis new.

[0029] It also appears that meltspun microfibers have sufficientstrength to form a barrier fabric without the need for any type ofsupporting scrim thus saving the additional materials, and cost of suchsupporting materials. While strength will be an important considerationto a buyer of such materials, stability will also be important. It hasbe found that microfibers may be meltspun at high speed that has lowshrinkage. A fabric having high barrier and permeability properties thatis strong and stable will have substantial value to makers and wearersof protective garments.

[0030] A potential key component for the success of the presentinvention to a nonwoven fabric may be in the creation of a hardenedmeltspun microfiber that is created without the steps of annealing anddrawing. In particular, it has been found that spinning microfibers athigh spinning speeds causes considerable changes in the properties ofthe fibers. Experiments were tested with 2GT polyester at a range ofspinning speeds to show the effect of the spinning speed differences onthe properties. As illustrated in the charts in FIGS. 3, 4, and 5, thetenacity dramatically increases, while the elongation to break and boiloff shrinkage dramatically decrease. The data is also tabulated in thefollowing Table A: TABLE A Spinning Speed (m/min) 3998 5029 5761 59436401 No. of Filaments 200 200 200 200 200 Fiber Size (denier) 0.5 0.50.5 0.5 0.5 Boil Off Shrinkage (%) 50.1 15.1 12.1 7.8 8.1 Tenacity(g/denier) 3.3 — 3.9 3.9 3.8 Elongation to Break (%) 49.0 — 33.0 31.833.2

[0031] It should be fairly clear that microfibers made at high spinningspeeds will obviate the need for annealing and drawing. The microfibersare strong and stable. Such high production speeds will be desirable forhigh productive rates of nonwoven fabrics although the handling of suchsmall fibers will be a challenge for any commercial installation.

[0032] In Tables B-D below, there is more data to confirm the foregoingdata. The next group includes round cross sections polyester as well asbi-lobe cross sections: TABLE B Spinning Speed (m/min) 2743 3200 36584115 4115 No. of Filaments 100 100 100 100 100 Fiber Size (denier) 0.70.7 0.7 0.63 0.55 Cross Section Round Round Round Round Round Boil OffShrinkage (%) 34 18 5.8 4.0 4.2 Tenacity (g/denier) 2.7 3.0 — 3.2 3.3Elongation to Break (%) 119 108 91 80 80

[0033] TABLE C Spinning Speed (m/min) 3658 4435 3200 3658 4115 No. ofFilaments 100 100 100 100 100 Fiber Size (denier) 0.63 0.55 0.72 0.780.48 Cross Section Round Round Bi-Lobe Bi-Lobe Bi-Lobe Boil OffShrinkage (%) 5.5 4.2 7.1 7.6 4.1 Tenacity (g/denier) 3.0 3.1 3.0 3.13.4 Elongation to Break (%) 86 70 102 96 75

[0034] TABLE D Spinning Speed (m/min) 3200 3200 No. of Filaments 68 100Fiber Size (denier) 0.78 0.53 Cross Section Round Round Boil OffShrinkage (%) 4.9 4.5 Dry Heat Shrinkage (%) 4.4 4.3 Tenacity (g/denier)3.3 3.0 Elongation to Break (%) 132 103

[0035] Clearly, it is an improvement in the art to provide fiber at ahigher rate with desired properties that are obtained without theordinary additional processing. It is particularly advantageous in thecontext of the improved nonwoven fabric.

[0036] In one aspect of the invention, the fabric may be subjected to acold nip to compress the fabric. Under microscopic analysis, the fibersin the compressed fabric appear to be stacked on one another withouthaving lost the basic cross sectional shape of the fiber. It appearsthat this is a relevant aspect of the invention since each fiber appearsto have not been distorted or substantially flattened which would closethe pores. As a result, the fabric has an increase in the barrierproperties as measured by hydrostatic head seems to maintain a high voidratio and low density and very high Frazier permeability.

[0037] From a macroscopic analysis, the inventive fabrics are generallycharacterized have a balance of tremendously high Frazier permeabilitywhile exhibiting substantial hydrostatic head pressures. For example insome test fabrics the initial hydrostatic head may be at a level that isabout 30 cm while the Frazier is above 65 m³/min-m². The Frazierpermeability and Hydrostatic head may be readily modified simply by coldcalendering the inventive fabric. After calendering, the hydrostatichead may be brought up to as much as 45 to 50 cm while the Frazierremains in excess of 25 m³/min-m². A fabric having high barrierproperties with high breathability is believed to be highly desirable asa protective fabric in the medical field and possibly many other fields.

[0038] While the description of the invention has thus far been relatedto meltspun fibers which are only recently being made in the sub-deniersizes; however, there may be other spinning technologies either nowdeveloped or yet to be invented that could provide suitable polymericfibers. The general range of preferred fibers have cross sectional sizesof between about 6 and about 90 μm² where fibers having a range fromabout 20 to about 70 μm² is more preferred and a range of about 33 toabout 54 μm² is most preferred. Fiber sizes are conventionally describedas denier or decitex. In the present circumstance, it is believed thatthe properties are achieved in part by a function of the physical sizeof the fibers. As denier and decitex relate to the weight of a longlength of fiber, the density of the polymer may create some misleadinginformation. For example, if two fibers have the same cross section, butone is made of polyethylene while the other comprises polyester, thepolyester would have a greater denier since it tends to be more densethan polyethylene. However, it can generally be regarded that thepreferred range of fiber denier is less than or nearly equal to about 1.

[0039] As noted above the fiber should be a hardened fiber. The crosssectional shape is not yet believed to be critical to the invention, butmost compact cross sections are presumed to be best as the pores willmost likely be small but not closed. Clearly, there may be someenhancements to the fabrics of the present invention by various crosssectional shapes of the fibers. At the same time, the fibers arepreferred to have sufficient tensile strength that a support layer isnot required. This is probably achieved by being composed of fibershaving a minimum strength of at least about 275 MPa. Such fiber shouldeasily provide sheet grab strengths in excess of 1 N/g/m² normalized forbasis weight. The fiber strength of the present invention willaccommodate most applications without reinforcement such as themeltblown layer in SMS. Melt blown fibers typically have tensilestrengths from about 26 to about 42 MPa due to the lack of polymerorientation in the fiber. In this application, hydrostatic headpressures are measured on the various sheet examples in an unsupportedmanner so that if the sheets do not comprise a sufficient number ofstrong fibers, the measurement is not attainable. Thus, unsupportedhydrostatic head pressure is a measure of barrier as well as anindication that the sheet has the intrinsic strength to support thehydrostatic head pressure.

[0040] It should be recognized that although the inventive fabric hasbeen characterized by hydrostatic head, that the small pores will make agood barrier for dry particulate materials. Thus, with the high Frazierpermeability that the fabric may be suitable for some filterapplications. It should be recognized that basis weight of the sheetmaterial will have some effect on the balance of hydrostatic head andpermeability. In most cases, it will be desirable from both an economicand productivity standpoint as well as property balance standpoint tohave the basis weight be about or below 75 g/m². However, there arepotential end uses where heavier and higher barrier sheet materialswould be desirable such as certain protective apparel applications, forexample. In such cases, the basis weight may be greater than about 70g/m² and could be quite heavy such 200 g/m², for example.

[0041] The preferred fiber would be any of a variety of polymers orcopolymers including polyethylene, polypropylene, polyester, and anyother melt spinnable fiber which would be less than approximately 1.2decitex per filament. The fiber would be a hard yarn which isconventionally fully drawn and annealed having strength and lowshrinkage. As noted above, fibers hardened by high speed melt spinningmay be suitable for the present invention. The fabric properties mayalso be modified by variations of the fiber cross sections.

[0042] A number of Examples of the present invention have been preparedas follows

EXAMPLES 1-37

[0043] Fabric samples were made with a lab batch wet-lay apparatus withmeltspun PET fiber cut to 5 mm. The fiber was manufactured by TeijenFibers and is commercially available. All samples were treated with anacrylic binder (Barriercoat 1708) to provide the sample with strengthand finished with a repellent finish (Freepel 114, Zonyl 8315, NaCl,Isopropyl Alcohol) to give hydrophobic properties. Fiber size is belowreported as decitex for round cross sectional fiber. As noted above, thefiber in the present invention need not necessarily be round. Thus, itmay be more clear to recognize that decitex is a measure of both polymerdensity and cross sectional area of the fibers. Thus, for a 0.333decitex (0.3 denier) PET fiber (2GT polyester) the cross sectional areais about 25 microns (μm²). A 0.867 decitex PET fiber will have a 65micron cross sectional area.

[0044] The data are tabulated below: TABLE I Ex. 1 Ex. 2 Ex. 3 Ex. 4Basis Weight (g/m²) 44.1 44.1 44.1 44.1 Fiber Size (decitex) 0.333 0.3330.333 0.333 Thickness (mm) 0.33 0.34 0.36 0.38 Frazier Permeability 27.729.3 32.0 36.6 (m³/min-m²) Hydrostatic Head (cm) 45 47 44 44.5Density(gm/cc) 0.1336 0.1287 0.1241 0.1158 Void (%) 90.18 90.54 90.8891.49

[0045] TABLE II Ex. 5 Ex. 6 Ex. 7 Ex. 8 Basis Weight (g/m² ) 44.1 44.144.1 44.1 Fiber Size (decitex) 0.333 0.333 0.333 0.333 Thickness (mm)0.38 0.41 0.48 0.56 Frazier Permeability 44.8 43.6 42.1 51.2 (m³/min-m²)Hydrostatic Head (cm) 40 40.5 39.5 38.5 Density (gm/cc) 0.1158 0.10860.0914 0.0789 Void (%) 91.49 92.02 93.28 94.19

[0046] TABLE III Ex. 9 Ex. 10 Ex. 11 Ex. 12 Basis Weight (g/m²) 44.144.1 54.2 64.4 Fiber Size (decitex) 0.333 0.333 0.333 0.333 Thickness(mm) 0.58 0.58 0.63 0.53 Frazier Permeability 45.1 56.4 46.6 25.3(m³/min-m²) Hydrostatic Head (cm) 41 34.33 35 46.5 Density (gm/cc)0.0755 0.0755 0.0855 0.1209 Void (%) 94.45 94.45 93.71 91.11

[0047] TABLE IV Ex. 13 Ex. 14 Ex. 15 Ex. 16 Basis Weight (g/m²) 64.443.1 43.4 53.6 Fiber Size (decitex) 0.333 0.867 0.867 0.867 Thickness(mm) 0.79 0.43 0.41 0.41 Frazier Permeability 38.1 73.8 65.2 50.0(m³/min-m²) Hydrostatic Head (cm) 38 28 31 32 Density (gm/cc) 0.08190.0998 0.1069 0.1319 Void (%) 93.98 92.66 92.14 90.30

[0048] TABLE V Ex. 17 Ex. 18 Ex. 19 Ex. 20 Basis Weight (g/m²) 54.2 62.063.4 50.56 Fiber Size (decitex) 0.867 0.867 0.867 0.11 Thickness (mm)0.46 0.51 0.46 0.18 Frazier Permeability 57.9 50.3 43.3 4.74 (m³/min-m²)Hydrostatic Head (cm) 29 30 33 72 Density (gm/cc) 0.1188 0.1223 0.1388Void (%) 91.27 91.01 89.79

[0049] TABLE VI Ex. 21 Ex. 22 Ex. 23 Ex. 24 Basis Weight (g/m²) 48.5349.55 71.27 75.34 Fiber Size(decitex) 0.11 0.11 0.11 0.11 Thickness (mm)0.20 0.20 0.23 0.30 Frazier Permeability 9.12 8.57 3.04 5.17 (m³/min-m²)Hydrostatic Head (cm) 73 60 99 77

[0050] TABLE VII Ex. 25 Ex. 26 Ex. 27 Ex. 28 Basis Weight (g/m²) 73.6452.60 55.32 52.60 Fiber Size (decitex) 0.11 0.33 0.33 0.33 Thickness(mm) 0.30 0.20 0.30 0.36 Frazier Permeability 4.86 15.14 25.69 31.62(m³/min-m²) Hydrostatic Head (cm) 63.5 48 43 38.5

[0051] TABLE VIII Ex. 29 Ex. 30 Ex. 31 Ex. 32 Basis Weight (g/m²) 70.9375.68 75.68 53.96 Fiber Size (decitex) 0.33 0.33 0.33 0.56 Thickness(mm) 0.23 0.38 0.56 0.20 Frazier Permeability 8.63 18.6 24.02 16.84(m³/min-m²) Hydrostatic Head (cm) 55.5 46.5 41.5 40.5

[0052] TABLE IX Ex. 33 Ex. 34 Ex. 35 Ex. 36 Basis Weight (g/m²) 54.6452.94 76.70 67.87 Fiber Size (decitex) 0.56 0.56 0.56 0.56 Thickness(mm) 0.30 0.38 0.25 0.38 Frazier Permeability 40.74 45.60 10.49 31.92(m³/min-m²) Hydrostatic Head (cm) 33 31 44 34

[0053] TABLE X Ex. 37 Basis Weight (g/m²) 76.02 Fiber Size (decitex)0.56 Thickness (mm) 0.56 Frazier Permeability 33.44 (m³/min-m²)Hydrostatic Head (cm) 32.5

EXAMPLES 38-40

[0054] Fabric samples 38-40 were “hand-made” using polypropylenecontinuous fibers with diameters as indicated in Table XI. The sampleswere hot pressed as at the Bonding temperatures as indicated in TableXI. TABLE XI Ex. 38 Ex. 39 Ex. 40 Basis Weight (g/m²) 59.3 48.1 51.9Fiber Size (μm) 20 20 14-18 Bonding Temp (° C.) 152 154 154 FrazierPermeability 75.0 60.0 288.3 (m³/min-m²) Hydrostatic Head (cm) 20.1 15.017.0

EXAMPLES 41 and 42

[0055] Fabric samples 41 and 42 were “hand-made” similar to Examples38-40 except that the fabric is made by using two plies of the hand-madesamples. The data from samples 41 and 42 are set forth in Table XII.TABLE XII Ex. 41 Ex. 42 Basis Weight (g/m²) 128.8 101.7 Fiber Size (μm)14-18 20 Bonding Temp (° C.) 154 154 Frazier Permeability 35.1 20.7(m³/min-m²) Hydrostatic Head (cm) 158.0 228.1

[0056] The data from Tables XI and XII clearly indicate that a uniquecombination of barrier and air permeability may be formed by theinventive fabric which is not found in other available nonwoven fabrics.The uses of such fabrics and structures may be exceptionally broad asthe combination or balance of properties has never really beenanticipated in a single fabric. Principally, the fabric may be used inspecial use apparel such as a medical gown for a surgeon. It would befor a single use to protect the surgeon or other medical personnel fromhazardous liquids such as contaminated body fluids. However, during along and intense operation, the medical personnel would not beoverheating but rather would be quite comfortable in a garment thatbreathes. After use, the garment would preferably be fully recyclable asit would be constituted of a single polymer which would be readilyrecycled back to constituent monomer as compared to other materialswhich are combinations of dissimilar polymers or wherein at least oneconstituent is not a recyclable polymer.

[0057] Although there are disclosed a number of examples related towetlay nonwoven fabrics and then discussion of fibers that may be spuninto strong, stable fibers without annealing and drawing, thecombination of both aspects of the invention into a nonwoven fabric madedirectly from strong, stable fiber as the fiber is spun and which avoidsthe need for annealing and drawing would be at least one preferredarrangement of the invention.

[0058] There are several additional aspects to preferred arrangements ofthe invention. The small denier fiber may be spun as a bicomponentconjugate fiber or multi-component conjugate fiber and split into finerfibers after the fibers are spun. One advantage of spinning conjugatefibers is higher potential production rates depending on the mechanismfor splitting the conjugate fibers. Each of the resulting split fibersmay have a pie shaped or other shaped cross section.

[0059] Another aspect is to provide bicomponent or polymers such assheath-core arrangements. A sheath-core bi-component fiber isillustrated in FIG. 4 where a fiber 80 is shown in cross section. Thesheath polymer 82 surrounds the core polymer 84 and the relative amountsof polymer may be adjusted so that the core polymer 84 may comprise moreor less than fifty percent of the cross sectional area. With thisarrangement, a number of attractive alternatives can be produced. Forexample, the sheath polymer 82 can be blended with pigments which arenot wasted in the core, thereby reducing the costs for pigments whileobtaining a suitably colored material. A hydrophobic material such as afluorocarbon may also be spun into the sheath polymer to obtain thedesired liquid repellency at minimal cost. An antimicrobial additive maybe suitable in some healthcare applications. Stabilizers may be providedfor a number of applications such as ultraviolet energy exposure, whereoutdoor exposure to sunlight may be one example. A static electricitydischarge additive may be used for applications where a build up ofelectricity is possible and undesirable. Another additives may besuitable such as a wetting agent to make the sheet material suitable asa wipe or absorbent or to allow liquids to flow through the fabric whilevery fine solids are collected in the fine pores of the sheet material.As the sheet material is proposed to be comprised of generallycontinuous filaments, the sheet material may be amenable as a wipehaving low linting characteristics.

[0060] A polymer having a lower melt point or melting temperature may beused as the sheath to so as to be amenable to melting during bondingwhile the core polymer does not soften. One very interesting example isa sheath core arrangement using 2GT polyester as the core and 3GTpolyester as the sheath. Such an arrangement would be suited forradiation sterilization such as e-beam and gamma ray sterilizationwithout degradation. Other combinations of multi-component fibers andblends of fibers may be envisioned. Various polymers present challengesand opportunities. The sheet material of the present invention maycomprise polyester (such as polyethylene teraphthalate, polypropyleneteraphthalate, and polybutylene teraphthalate) combinations and blendsof polyester, nylon, a polyolefin such as polyethylene andpolypropylene, and even elastomeric polymers.

[0061] The foregoing description and drawings were intended to explainand describe the invention so as to contribute to the public base ofknowledge. In exchange for this contribution of knowledge andunderstanding, exclusive rights are sought and should be respected. Thescope of such exclusive rights should not be limited or narrowed in anyway by the particular details and preferred arrangements that may havebeen shown. Clearly, the scope of any patent rights granted on thisapplication should be measured and determined by the claims that follow.

1. A flexible sheet material having a Frazier permeability of at leastabout 70 m³/min-m² and an unsupported hydrostatic head of at least about15 cm.
 2. The flexible sheet material according to claim 1 wherein thehydrostatic head is at least about 20 cm.
 3. A flexible sheet materialhaving a Frazier permeability of at least about 28 m³/min-m² and anunsupported hydrostatic head of at least about 30 cm.
 4. A flexiblesheet material having a Frazier permeability of at least about 15m³/min-m² and a hydrostatic head of at least about 40 cm.
 5. A flexiblesheet material having a Frazier permeability of at least about 1m³/min-m² and a hydrostatic head of at least about 80 cm.
 6. A flexiblesheet material comprised of meltspun nonwoven fibers having an averagelength of at least about 4 cm and wherein a substantial majority of thefibers have a cross section of less than about 70 square microns and theaverage fiber strength is at least 275 N/mm².
 7. A flexible sheetmaterial formed of nonwoven fibers where in the sheet has a basis weightof at least about 13 g/m² up to about 75 g/m², and wherein substantiallyall of the fibers are meltspun fibers, a substantial majority by weightof the fibers have a cross section of less than about 90 square microns,and wherein the sheet material has a Frazier permeability is at leastabout 1 m³/min-m² and a hydrostatic head of at least about 25 cm.
 8. Thesheet material according to claim 7 wherein the hydrostatic head is atleast 30 cm.
 9. The sheet material according to claim 7 wherein thehydrostatic head is at least 40 cm.
 10. The sheet material according toany one of claims 5, 6, and 7 wherein the Frazier permeability is atleast about 5 m³/min-m².
 11. The sheet material according to any one ofclaims 5 and 7 wherein the Frazier permeability is at least about 10m³/min-m².
 12. The sheet material according to any one of claims 5 and 7wherein the Frazier permeability is at least 15 m³/min-m².
 13. The sheetmaterial according to any one of claims 4, 5 and 7 wherein the Frazierpermeability is at least 25 m³/min-m².
 14. The sheet material accordingto any one of claims 3, 4, 5 and 7 wherein the Frazier permeability isat least 35 m³/min-m².
 15. The sheet material according to any one ofclaims 3, 4 and 7 wherein the Frazier permeability is at least about 45m³/min-m².
 16. The sheet material according to any one of claims 3, 4,and 7 wherein the hydrostatic head is at least 50 cm.
 17. The sheetmaterial according to any one of claims 3, 4, and 7 wherein thehydrostatic head is at least 60 cm.
 18. The sheet material according toany one of claims 1, 3, 4, and 5 wherein the sheet material is comprisedof fibers wherein the average fiber size is less than about 90 μm². 19.The sheet material according to any one of claims 1, 3, 4, 5, and 7wherein the sheet material is comprised of fibers wherein the averagefiber size is less than about 75 μm².
 20. The sheet material accordingto any one of claims 1, 3, 4, 5, 6 and 7 wherein the sheet material iscomprised of fibers wherein the average fiber size is less than about 60μm².
 21. The sheet material according to any one of claims 1, 3, 4, 5,and 7 wherein the sheet material is comprised of fibers having a minimumfiber strength of about 275 newtons per square millimeter.
 22. The sheetmaterial according to any one of claims 1, 3, 4, 5, 6, and 7 wherein thesheet has a grab tensile strength of at least about 1 N/g/m².
 23. Thesheet material according to any of claims 1, 3, 4, 5, 6, and 7 whereinthe sheet material is comprised of fibers and wherein the majority offibers have a boil off shrinkage of less than ten percent.
 24. The sheetmaterial according to any of claims 1, 3, 4, 5, 6, and 7 wherein thesheet material is comprised of fibers which are split fibers from largerconjugate melt spun fibers.
 25. The sheet material according to any ofclaims 1, 3, 4, 5, 6, and 7 wherein the sheet material is comprised offibers, and at least a portion of the fibers are formed of at least twoseparate component polymers.
 26. The sheet material according to claim25 wherein one of said components overlies the other in a sheath-corearrangement.
 27. The sheet material according to claim 26 wherein thesheath component of the fibers includes at least one additive blendedinto the polymer.
 28. The sheet material according to claim 27 whereinthe additive is a hydrophobic additive to repel liquids.
 29. The sheetmaterial according to claim 28 wherein the additive is a fluorocarbon.30. The sheet material according to claim 27 wherein the additive is astabilizer.
 31. The sheet material according to claim 30 wherein thestabilizer is a stabilizing agent for ultraviolet energy exposure. 32.The sheet material according to claim 28 wherein the additive is awetting agent to cause mechanical absorption of liquids into the fabric.33. The sheet material according to claim 28 wherein the additiveprovides a color to the fibers and fabric.
 34. The sheet materialaccording to claim 28 wherein the additive reduces the buildup of staticelectricity in the fabric.
 35. The sheet material according to claim 28wherein the additive is an antimicrobial agent.
 36. The sheet materialaccording to claim 27 wherein the polymer comprising the sheath has alower melting temperature than the polymer comprising the core.
 37. Thesheet material according to claim 27 wherein the polymer comprising thesheath does not substantially degrade from exposure to radiationsterilization processing.
 38. The sheet material according to any ofclaims 1, 3, 4, 5, 6, and 7 wherein the sheet is comprised of fibers anda first portion of the fibers is comprised of a first polymer and asecond portion is formed of a second polymer, wherein one of said firstand second polymers melts at a lower temperature than the other tofacilitate thermal bonding.
 39. The sheet material according to any ofclaims 1, 3, 4, 5, 6, and 7 wherein the sheet is comprised of fibers andthe fibers comprise polyester polymer.
 40. The sheet material accordingto claim 39 wherein the fibers are comprised of polyethyleneterephthalate polymer.
 41. The sheet material according to claim 39wherein the fibers are comprised of polypropylene terephthalate polymer.42. The sheet material according to claim 39 wherein the fibers arecomprised of polybutylene terephthalate polymer.
 43. The sheet materialaccording to claim 39 wherein the fibers are comprised of polyester withan additional polymer blended with the polyester polymer.
 44. The sheetmaterial according to any of claims 1, 3, 4, 5, 6, and 7 wherein thesheet is comprised of fibers and the fibers comprise nylon polymer. 45.The sheet material according to any of claims 1, 3, 4, 5, 6, and 7wherein the sheet is comprised of fibers and the fibers comprisepolyethylene polymer.
 46. The sheet material according to any of claims1, 3, 4, 5, 6, and 7 wherein the sheet is comprised of fibers and thefibers comprise polypropylene polymer.
 47. The sheet material accordingto any of claims 1, 3, 4, 5, 6, and 7 wherein the sheet material iscomprised of fibers and the fibers are comprised of elastomeric polymer.48. The sheet material according to any of claims 1, 3, 4, 5, 6, and 7wherein the sheet is comprised of fibers and the fibers comprise a blendof different polymers.
 49. The sheet material according to any of claims1, 3, 4, 5, 6, and 7 wherein the sheet is comprised of fibers and thefibers comprise at least one additive blended into the polymer.
 50. Thesheet material according to claim 49 wherein the additive is ahydrophobic additive to repel liquids.
 51. The sheet material accordingto claim 49 wherein the additive is a fluorocarbon.
 52. The sheetmaterial according to claim 49 wherein the additive is a stabilizer. 53.The sheet material according to claim 52 wherein the stabilizer is astabilizing agent for ultraviolet energy exposure.
 54. The sheetmaterial according to claim 49 wherein the additive is a wetting agentto increase mechanical absorption of liquids into the fabric.
 55. Thesheet material according to claim 49 wherein the additive provides acolor to the fibers and fabric.
 56. The sheet material according toclaim 49 wherein the additive reduces the buildup of static electricityin the fabric.
 57. The sheet material according to claim 49 wherein theadditive is an antimicrobial agent.
 58. The sheet material according toany of claims 1, 3, 4, 5, 6, and 7 wherein the sheet material is formedof fibers with a repellent finish applied thereon.
 59. The sheetmaterial according to claim 58 wherein said repellent finish comprises afluorocarbon.
 60. The sheet material according to any of claims 1, 3, 4,5, 6, and 7 wherein the sheet material is comprised of melt extrudedgenerally continuous filament polymer fibers.
 61. The sheet materialaccording to claim 60 wherein the fibers are ultrasonically bondedtogether.
 62. The sheet material according to claim 60 wherein thefibers which are thermally bonded together.
 63. The sheet materialaccording to claim 60 wherein the sheet material is comprised of fiberswhich are adhesively bonded together.
 64. The sheet material accordingto any of claims 1, 3, 4, 5, 6, and 7 wherein the material has a crosssectional void percentage of at least about 85 percent.
 65. The sheetmaterial according to claim 64 wherein the material has a crosssectional void percentage of at least about 89 percent.
 66. The sheetmaterial according to any of claims 1, 3, 4, 5, 6, and 7 wherein thepolymer does not substantially degrade due to exposure to radiationsterilization processing.
 67. The sheet material according to claim 66wherein the polymer does not substantially degrade due to exposure togamma radiation.
 68. The sheet material according to claim 66 whereinthe polymer does not substantially degrade due to exposure to e-beamradiation.
 69. The sheet material according to any of claims 1, 3, 4, 5,6, and 7 wherein the sheet material is comprised of layers of fibersforming a nonwoven sheet and wherein all of the layers are direct laidmeltspun generally continuous fibers.
 70. The sheet material accordingto any of claims 1, 3, 4, 5, and 6 wherein the basis weight is greaterthan 13 grams per square meter and less than 100 grams per square meter.71. The sheet material according to claim 5 wherein the basis weight isgreater than 65 grams per square meter and less than 250 grams persquare meter.
 72. A radiation sterilization stable sheath-corebi-component fiber suited for making a thermally bonded nonwoven fabricwherein the core polymer is polyethylene teraphthalate and the sheathfiber is polypropylene teraphthalate.
 73. The radiation sterilizationstable sheath-core bi-component fiber according to claim 72 wherein thesheath polymer includes pigment blended therein and the core polymer isgenerally free of pigment.
 74. The radiation sterilization stablesheath-core bi-component fiber according to claim 73 wherein the sheathpolymer further includes a fluorocarbon blended therein.
 75. Theradiation sterilization stable sheath-core bi-component fiber accordingto claim 73 wherein the average cross sectional area of the fiber isless than 90 square microns.